There is not enough information to suggest any spring rate as the frame you are mounting the shock is very important (linkage curve/rate is very important). There are calculators to estimate what spring might be the best. For example: TF Tuned Shox - Spring Calculator

But these calculators are only an estimation (as there are a number of other factors to consider that many calculators don't take into account) and not precise. So you might have to buy a couple of springs to find the right one for you.